Inside the left-most cave of Agate Village is an old woman who'll teach your POKeMON an attack much like the tutors in Fr/Lg/E. These are one-time only however so use them wisely. The ones in bold are exclusive to POKeMON XD.

A little ways into the game you'll learn of Poke Spots and be able to visit them. In these three places wild POKeMON can be battled and caught. In order to attract them you'll need to place Poke Snacks in the center and leave for a little while.

The Spot Monitor attachment to your P*DA will alert you when a wild POKeMON appears and where its at. After it goes off you have only a limited amount of time to reach the spot where a POKeMON is at. The more snacks you placed, the longer time you'll have to get there.

Rock Poke Spot:

Common - Sandshrew Level - 10 to 22

Uncommon - Gligar Level - 10 to 22

Rare - Trapinch Level - 10 to 22

Cave Poke Spot:

Common - Zubat Level - 10 to 22

Uncommon - Aron Level - 10 to 22

Rare - Wooper Level - 10 to 22

Oasis Poke Spot:

Common - Hoppip Level - 10 to 22

Uncommon - Phanpy Level - 10 to 22

Rare - Surskit Level - 10 to 22

Notes:

After Bonsly runs away from you at the S.S. Libra it will show up at one of the Poke Spots. Approach it with slow steps and you'll "catch" it. Then return to the ONBS building in Pyrite Town and return Bonsly to its trainer for the nice prize of a Bonsly Card to use at Battle Bingo once you've beaten all of the other cards.

On occasion you may find a Munchlax chomping away at your Poke Snacks. Its owner will show up and apologize to you and give you 10 Poke Snacks and a free sample of the herbal medicines she sells in Gateon Ports POKeMON Mart.

Duking:

Duking would like you to trade him each of the rarely found POKeMON at each spot. He'll be in the back room of his Pyrite Town home waiting for you to bring them to him.

Trapinch for Meditite
Wooper for Larvitar
Surskit for Shuckle

Please note that if you've completed the game and unlocked trading you can import the needed POKeMON from the GBA games and trade them to him rather than searching for them at the Poke Spots.

Pre-Order Bonus

For pre-ordering POKeMON XD in the US you get a custom vinyl skin covering for your Nintendo Gamecube.